Method for the manufacturing of an aluminium-magnesium-lithium alloy product

ABSTRACT

Method for manufacturing of an aluminium-magnesium-lithium product, comprising the steps of subsequently: (a) providing an aluminium alloy consisting of (in weight %): Mg 3.0-6.0, Li 0.4-3.0, Zn up to 2.0, Mn up to 1.0, Ag up to 0.5, Fe up to 0.3, Si to 0.3, Cu up to 0.3, 0.02-0.5 selected from the group consisting of (Sc 0.010-0.40, Hf 0.010-0.25, Ti 0.010-0.25, V 0.010-0.30, Nd 0.010-0.20, Zr 0.020-0.25, Cr 0.020-0.25, Y 0.005-0.20, Be 0.0002-0.10), balance consisting essentially of aluminium and incidental elements and impurities; (b) casting the aluminium alloy into an ingot; (c) preheating the ingot; (d) hot rolling the preheated ingot to a hot worked intermediate product; (e) cold rolling the hot worked intermediate product to a rolled product in both the length and in the width direction with a total cold rolling reduction of at least 15%; (f) solution heat treating the cold rolled product in the temperature range of 465 to 565° C. for a soaking time in the range of 0.15 to 8 hours; (g) cooling the solution heat treated product from the solution heat treatment temperature to below 150° C. with a cooling rate of at least 0.2° C./sec; (h) ageing the cooled product to provide a sheet or thin plate product having a minimum yield strength of 260 MPa or more and a minimum tensile strength of 400 MPa or more in at least the L- and LT-direction, a minimum yield strength of 230 MPa or more and a minimum tensile strength of 380 MPa or more in the 45° to the L-direction, and further having a minimum T-L fracture toughness K co  of 80 MPa.{square root}m or more for 400 mm wide CCT-panels.

FIELD OF THE INVENTION

[0001] The invention relates to a method for the manufacturing of analuminium-magnesium-lithium product with less anisotropy of mechanicalproperties, and further the invention relates to the use of the obtainedproduct for structural components of aircraft.

[0002] For the purpose of this invention sheet material is to beunderstood as a rolled product having a thickness of not less than 1.3mm (0.05 inch) and not more than 6.3 mm (0.25 inch). See also AluminiumStandards and Data, Aluminium Association, Chapter 5 Terminology, 1997.Thin plate material is to be understood as a rolled product having athickness of not less than 6.3 mm and not more than 12 mm.

[0003] A cast ingot or slab is a three dimensional object having bydefinition a length is (normally the casting direction in case of(semi)-continuous casting), a width and a thickness, whereby the widthis equal to or larger than the thickness.

DESCRIPTION OF THE RELATED ART

[0004] It is well known that adding lithium as an alloying element toaluminium alloys results in beneficial mechanical properties.Aluminium-lithium alloys exhibit improvements in stiffness and strengthwhile reducing density to a significant extent. Consequently, thesetypes of alloys have utility as structural materials in aircraft andaerospace applications. Examples of known aluminium-lithium alloysinclude the British alloy AA8090, the American alloys AA2090 and AA2091,and the Russian alloy 01420.

[0005] Problems exist both with aluminium-lithium alloys and thealuminium-magnesium-lithium alloys, particularly in the anisotropy ofmechanical properties and fracture toughness. Fracture toughness valuesin the T-L direction tend to be significantly lower than fracturetoughness values in the main direction, viz. the L-T direction.

[0006] Some other disclosures of Al-Li alloys found in the prior artliterature will be mentioned below.

[0007] WO-92/03583 proposes an alloy useful in aircraft and airframestructures which has low density. The composition is, in wt. %: Mg0.5-10.0, preferably 7.0-10.0 Li 0.5-3.0, preferably 1.0-1.5 Zn 0.1-5.0,preferably 0.3-1.0 Ag 0.1-2.0, preferably 0.3-1.0 balance aluminium,

[0008] and with the proviso that the total amount of alloying elementsdoes not exceed 12.0, and with the further proviso that when Mg rangesfrom 7.0 to 10.0, Li cannot exceed 2.5% and Zn cannot exceed 2.0%.

[0009] Said alloy includes a mandatory amount of silver. In order themanufacture rolled product of this aluminium alloy standard processingparameters have been applied.

[0010] GB-A-2146353 proposes an alloy having a high electricalresistance and an excellent formability, useful in structures sufferingthe action of high magnetic field, nuclear fusion reactors or the like.The composition is, in wt. %: Mg 1.0-8.0, preferably 2.0-7.0 Li 0.05-1.0

[0011] at least one element selected from the group consisting of: Ti0.05-0.20 Cr 0.05-0.40 Zr 0.05-0.30 V 0.05-0.35 W 0.05-0.30 Mn 0.05-2.0balance aluminium and incidental impurities.

[0012] Further, Bi in the range of 0.05 to 0.50 wt. % may be containedin this alloy. In order the manufacture rolled product of this aluminiumalloy standard processing parameters have been applied.

[0013] DE-A-1558491 discloses the Russian alloy development for their1420 alloy referenced above, the alloy contains, in wt. %: Mg 4-7 Li1.5-2.6 Zr 0.05-0.3 or alternatively Ti 0.05-0.15 Mn 0.2-1.0 balancealuminium and impurities.

[0014] JP-A-61227157 discloses an Al-Li and a method of its manufacture,the disclosed alloy consists of, in wt. %: Li 1.0-5.0

[0015] one or more selected from the group consisting of: Zr 0.05-0.3 Cr0.05-0.3 Mn 0.05-1.5 V 0.05-0.3 Ti 0.005-0.1 balance aluminium

[0016] In order the manufacture rolled product of this aluminium alloystandard processing parameters have been applied.

SUMMARY OF THE INVENTION

[0017] In view of the drawbacks in aluminium-lithium alloys and inaluminium-magnesium-lithium alloys with respect to fracture toughness, aneed has developed to provide a method of improving the T-L fracturetoughness for these types of alloys. In response to this need, thepresent invention provides a method therefor which significantlyincreases the fracture toughness of aluminium-magnesium-lithium alloysin the T-L direction, thereby improving their suitability for morecommercial applications, in particular for use as structural componentsin aircraft.

[0018] In accordance with the invention there is provided in a methodfor the manufacturing of an aluminium-magnesium-lithium product withless anisotropy of mechanical properties, comprising the steps ofsubsequently:

[0019] (a) providing an aluminium alloy consisting of (in weight %): Mg3.0-6.0 Li 0.4-3.0 Zn up to 2.0 Mn up to 1.0 Ag up to 0.5 Fe up to 0.3Si up to 0.3 Cu up to 0.3

[0020] 0.02-0.5 selected from the group consisting of (Sc 0.010-0.40, Hf0.010-0.25, Ti 0.010-0.25, V 0.010-0.30, Nd 0.010-0.20, Zr 0.020-0.25,Cr 0.020-0.25, Y 0.005-0.20, and Be 0.0002-0.10), and balance consistingessentially of aluminium and incidental elements and impurities;

[0021] (b) casting the aluminium alloy into an ingot;

[0022] (c) preheating the ingot;

[0023] (d) hot rolling the preheated ingot to a hot worked intermediateproduct;

[0024] (e) cold rolling the hot worked intermediate product to a rolledproduct in both the length and in the width direction with a total coldrolling reduction of at least 15%;

[0025] (f) solution heat treating the cold rolled product in thetemperature range of 465 to 565° C. for a soaking time in the range of0.15 to 8 hours;

[0026] (g) cooling the solution heat treated product from the solutionheat treating temperature to below 150° C. with a cooling rate of atleast 0.2° C./sec;

[0027] (h) ageing the cooled product to provide a sheet or thin plateproduct having a minimum yield strength of 260 MPa or more and a minimumtensile strength of 400 MPa or more in at least the L- and LT-direction,a minimum yield strength of 230 MPa or more and a minimum tensilestrength of 380 MPa or more in the 45° to the L-direction, and furtherhaving a minimum T-L fracture toughness K_(co) of 80 MPa.{square root}mor more for 400 mm wide Centre Cracked Fracture Toughness testpanels(CCT-panels).

[0028] With the method in accordance with the invention it is nowpossible to provide a sheet product or a thin plate product of theindicated type having the mechanical properties as set out, whichproperties are much more isotropic than manufactured in a coilproduction route. In particular this method allows for an improvement ofthe relevant properties in the T-L direction of the obtained product.And a further advantage of this method is that it allows for theproduction of much wider sheet products, for example up to 2.5 meterwide, in comparison with conventional coil production routes.

[0029] In an embodiment of the method in accordance with the inventionthe obtained product may be provided with a cladding. Such clad productsutilise a core of the aluminum-magnesium-lithium base alloy as set outin more detail below and a cladding on at least one side of the core,which cladding is usually of higher purity (higher percentage aluminiumthan in the core) and which, in particular, enhance appearance andcorrosion protects the core. The cladding includes, but is not limitedto, essentially unalloyed aluminium or aluminium containing not morethan 0.1 or 1% of all other elements. Aluminium alloys herein designated1xxx-type series include all Aluminium Association (AA) alloys,including the sub-classes of the 1000-type, 1100-type, 1200-type and1300-type. In addition, AA alloy 7072 containing zinc (0.8 to 1.3%) canserve as the cladding and alloys of the AA6000-series alloys, such as6003 or 6253, which contain typically more than 1% of alloyingadditions, can serve as cladding. Other alloys could also be useful ascladding as long as they provide in particular sufficient overallcorrosion protection to the core alloy. The clad layer or layers areusually much thinner than the core, each constituting 0.5 to 15 or 20 orpossibly 25% of the total composite thickness. A cladding layer moretypically constitutes around 0.5 to 12% of the total compositethickness.

[0030] The preheating of the cast ingot prior to hot rolling is usuallycarried out at a temperature in the range of 360 to 500° C. in single orin multiple steps. In either case, preheating decreases the segregationof alloying elements in the material as cast and dissolves solubleelements, such as Li. If the treatment is carried out below 360° C., theresultant homogenisation effect is inadequate. Furthermore, due tosubstantial increase in deformation resistance of the ingot, industrialhot rolling is difficult for temperatures below 360° C. The preferredtime of the above treatment is between 1 and 24 hours, preferablybetween 5 and 20 hours, and more preferably between 8 and 15 hours.Preferably the preheating is carried out at a temperature in the rangeof 400 to 470° C., more preferably of 410 to 450° C., and mostpreferably of 420 to 440° C.

[0031] Typically, prior to hot rolling the rolling faces of both thecladded and the non-cladded products are scalped in order to removesegregation zones near the cast surface of the ingot.

[0032] The hot rolling procedure of the method in accordance with theinvention involves preferably hot rolling of the preheated ingot in boththe length and width directions. During the hot rolling process rollingdirections can be changed alternatively more than once. The hot rollingis preferably carried out in the temperature range of 270 to 470° C. Ithas been found beneficial for the properties of the final product ifafter the final hot rolling step the product has a temperature above270° C., preferably above 300° C., and more preferably above 330° C.After the initial first hot rolling step the intermediate hot rolledproduct is preferably reheated to a temperature in the range of 360 to470° C. for 1 to 24 hours, and more preferably in the range of 410 to450° C., and most preferably of 420 to 440° C. A more preferred soaktime is in the range of 5 to 20 hours and more preferably in the rangeof 7 to 15 hours. This reheat treatment is repeated for each followingstep of hot rolling until the desired intermediate gauge is obtained.Using this hot rolling practice a further improvement of the mechanicalproperties is obtained as is a more isotropic structure of the finalproduct.

[0033] When necessary during the hot rolling process in accordance withthe invention the intermediate product can be cut into sub-products asto allow for hot rolling in both the length and width directions.

[0034] Preferably the hot rolled intermediate product is annealed priorto cold rolling to enhance workability. The annealing treatment ispreferably carried out at a temperature in the range of 360 to 470° C.and more preferably of 380 to 420° C. The soak time for annealing is inthe range of 0.5 to 8 hours, and preferably of 0.5 to 3 hours. Theannealed intermediate product is allowed to cool down to below 150° C.,preferably by using air cooling.

[0035] To produce the rolled sheet product in accordance with theinvention, the product is cold worked by means of cold rolling theproduct in both the length and in the width direction to the finaldesired product gauge, comprising a thickness reduction of at least 15%.A practical maximum thickness reduction during cold rolling is about 90%because of cracking of the sheet or thin plate without interanneal.Preferably the cold rolling degree is 20 to 50% at each step, andpreferably 20 to 40% at each step. Using a cold rolling practice as setout above in particular an improvement in the reduction of anisotropyhas been obtained in the mechanical properties, and more in particular abetter balance has been obtained in the 45° to the L-direction for theyield strength, the tensile strength and the elongation.

[0036] During cold rolling the rolled product may be subjected to aninterannealing treatment or intermediate annealing to improveworkability of the cold rolled product. Interannealing is preferablycarried out at a temperature in the range 300 to 500° C., morepreferably of 350 to 450° C., and most preferably of 380 to 410° C. Thesoak time for interannealing is in the range of 0.5 to 8 hours, andpreferably of 0.5 to 3 hours, after which the product is allowed to cooldown by air cooling.

[0037] The cold rolled sheet product in accordance with the invention isthen solution heat treated typically at a temperature in the range of465 to 565° C., preferably of 490 to 540° C., for a soaking time in therange of 0.15 to 8 hours, preferably for a soaking time of 0.5 to 3hours, and more preferably of 0.8 to 2 hours, during which the excessivephases dissolve to the maximum extent possible at that temperature.

[0038] To further provide for the desired strength and fracturetoughness necessary to the final product and to the operations informing that product, the product should be cooled to below 150° C. byusing a cooling rate of at least 0.2° C./sec, and preferably a coolingrate of at least 1° C./sec, typically by means of fast air cooling. Withthe combination of the relatively high soaking temperature andrelatively long soaking times and the indicated cooling rates animprovement is obtained in the desirable mechanical properties, inparticular this treatment is beneficial for the fracture toughnessK_(co) and for the elongation of the final product. It has also beenfound that the product obtained is essentially free from Type-ALüder-lines. And further the thermal stability of the product obtainedis improved.

[0039] After cooling the annealed product and prior to the artificialageing the product may be stretched, preferably at room temperature, anamount not greater than 3% of its original length or otherwise worked ordeformed to impart to the product a working effect equivalent tostretching not greater than 3% of its original length. Preferably thestretching is in a range of 0.3 to 2.5%, and more preferably of 0.5 to1.5% of its original length. The working effect referred to is meant toinclude rolling and forging as well as other working operations. It hasbeen found that by stretching the product of this invention the residualstresses therein are relieved and the flatness of the product isimproved, and also the ageing response is improved.

[0040] A suitable artificial ageing process in the method according tothis invention is giving in the international patent application no.WO-99/15708, which is being incorporated here by reference.

[0041] It should be mentioned here that a method is known from U.S. Pat.No. 4,151,013 to provide Al-Mg alloys sheets having magnesium in therange of 2 to 8% and the sheet being free from Type-A Lüder-lines afterstretching, comprising the steps of:

[0042] (a) heating the sheet to a temperature in the range of 455-565°C., (850 to 1050° F.) preferably in the range of 480-510° C. (900 to950° F.) for a soaking time of 0.5 to 10 minutes;

[0043] (b) cooling the sheet to below 175° C. (350° F.) with apredetermined cooling rate Q;

[0044] (c) stretching the sheet for 0.25 to 1% of its original length.

[0045] However, this document does not mention the use of this methodfor Al-Mg-Li alloys, and further it does not mention that with a longersoaking time in the range of 0.15 to 8 hours as set out in the methodaccording to the present invention also the Type-A Lüder-lines can beavoided and further that an improvement in the values for the fracturetoughness K_(co) and elongation of the final product may be obtained.Nor has it been mentioned that an improvement in the resistance to crackpropagation can be obtained.

[0046] After the product has been worked and annealed, it may be aged toprovide the combination of strength and fracture toughness andresistance to crack propagation which are so highly desired in aircraftmembers. The product may be naturally aged, typically at ambienttemperatures, and alternatively the product may be artificially aged toprovide the combination. This can be accomplished by subjecting thesheet or shaped product to a temperature in the range of 65 to 205° C.for a sufficient period of time to further increase the yield strength.

[0047] Further, it will be noted that the product formed in accordancewith the invention may be subjected to any of the typical underageingtreatments well known in the art. Also, while reference has been madeherein to single ageing steps, multiple ageing steps, such as two orthree ageing steps, are contemplated and stretching of its equivalentworking may be used prior to or even after part of such multiple ageingsteps.

[0048] In a preferred embodiment of the method in accordance with theinvention the obtained product has a minimum T-L fracture toughnessK_(co) of 90 MPa.{square root}m or more for 400 mm wide CCT-panels, andmore preferably of 95 MPa.{square root}m or more. In American basedliterature K_(co) of an material is often referred to as K_(co) or asapparent fracture toughness.

[0049] In a preferred embodiment of the method in accordance with theinvention the obtained product has a minimum tensile strength of 430 MPaor more in at least the L- and LT-direction, and more preferably aminimum of 450 MPa or more in these indicated directions. The preferredminimum tensile strength in the 45° to the L-direction is 390 MPa ormore, and more preferably 400 MPa or more.

[0050] In a preferred embodiment of the method in accordance with theinvention the obtained product has a minimum yield strength of 300 MPaor more in at least the L- and LT-, direction, and more preferably aminimum of 315 MPa or more, and most preferably of 330 MPa or more inthese indicated directions. The preferred minimum yield strength in the45° to the L-direction is 250 MPa or more, and more preferably 260 MPaor more, and more preferably of 270 MPa or more.

[0051] In a further embodiment of the method in accordance with theinvention the obtained product has a minimum yield strength of 400 MPaor more in the L-direction and a minimum yield strength of 370 MPa ormore in the LT-direction and a minimum yield strength of 330 MPa or morein the 45° to the L-direction.

[0052] The reasons for the limitations of the alloying elements of thealuminium-magnesium-lithium based product obtained by the methodaccording to the present invention are described below. All compositionpercentages are by weight.

[0053] Mg is the primary strengthening element in the product withoutincreasing density. Mg levels below 3.0% do not provide the requiredstrength and when the addition exceeds 6.0% severe cracking may occurduring the casting and hot rolling of the product. The preferred levelof Mg is between 4.3 to 5.5%, and more preferably of 4.7 to 5.3%, as acompromise between fabricability and strength.

[0054] Li is also an essential alloying element and to provide theproduct with a low density, high strength, good weldability, and a verygood natural ageing response. The preferred Li level is in the range of1.0 to 2.2%, more preferably of 1.3 to 2.0%, and most preferably of 1.5to 1.8%, as a comprise between fabricability and strength.

[0055] Zinc as an alloying element is may be present in the productaccording to this invention to provide improved precipitation hardeningresponse and corrosion performance. Zinc levels above 1.5% do notprovide good welding performance, and further increases density. Thepreferred level of zinc is 0.05-1.5%, and more preferably the level isbetween 0.2-1.0%.

[0056] Mn may be present in a range of up to 1.0%. The preferred levelif Mn is in the range of 0.02 to 0.5%, and more preferably in the rangeof 0.02 to 0.25%. In these ranges the added manganese will aid tocontrol the grain structure.

[0057] Cu is preferably not added to the product since it deterioratescorrosion resistance, although it is known that it can increasemechanical properties significantly. The Cu level should not exceed0.3%, while a preferred maximum is 0.20%, and more preferably themaximum level is 0.05%.

[0058] Sc may be present in range of up to 0.4% to improve the strengthof the product and to improve the weldability of the product by reducinghot crack sensitivity during welding, it will increase therecrystallisation temperature and improves the ability to control thegrain structure. The preferred range is from 0.01% to 0.08%, and morepreferably from 0.02 to 0.08%, as a compromise between strength andfabricability. Elements having similar effect, such as neodymium, ceriumand yttrium, or mixtures thereof, can be used, either instead of, or inaddition to, scandium, without changing the essence of the productaccording to this invention.

[0059] Zr is preferably added as a recrystallisation inhibitor and ispreferably present in a range of 0.02 to 0.25%, more preferably in arange of 0.02 to 0.15%, and most preferably of 0.05 to 0.12%. Althoughother grain refiners can be used for aluminium-magnesium-lithium alloys,zirconium proved to be the most effective one for this type of alloys.Elements having similar effect, such as chromium, manganese, hafnium,titanium, boron, vanadium, titanium diboride, or mixtures thereof, canbe used, either instead of, or in addition to, zirconium, withoutchanging the essence of the product according to this invention.

[0060] The expensive alloying element silver, which is frequently usedin this type of alloys, may be added. Although it can be added in theusual range of up to about 0.5%, and preferably in the range of up to0.3%, it may not result in a significant increase in properties, but mayenhance the ageing response, which is extremely useful for welding.

[0061] Iron and silicon can each be present in maximums up to a total of0.3%. It is preferred that these impurities be present only in traceamounts, limiting the iron to a maximum of 0.15% and the silicon to amaximum of 0.12%, and more preferably to maximums of 0.10% and 0.10%,respectively.

[0062] The trace elements sodium and hydrogen are also thought to beharmful to the properties (fracture toughness in particular) ofaluminium-magnesium-lithium alloys and should be held to the lowestlevels practically attainable, for example on the order of 15 to 30 ppm(0.0015-0.0030%) for the sodium and less than 15 ppm (0.0015%) andpreferably less than 1.0 ppm (0.0001%) for the hydrogen. The balance ofthe alloy, of course, comprises aluminium and incidental impurities.Typically each impurity element is present at 0.05% maximum, and thetotal of impurities is 0.15% maximum.

[0063] The invention further consists in the use of thealuminium-magnesium-lithium product obtained by the method of this forstructural components of aircraft such as aircraft skin, and also forthe manufacture of aircraft lower wing skins, and can be further usedfor the skin of aircraft fuselages.

EXAMPLES

[0064] The invention will now be illustrated by several non-limitatitiveexamples.

Example 1

[0065] Three ingots have been produced on an industrial scale, of whichthere are two manufactured in accordance with the invention and one ismanufactured for comparison. Three ingots A, B and C (compositions arelisted in Table 1) having dimensions 350×1450×2500 mm have beenpreheated to 395° C. for about 8 hours, and then hot rolled in theirwidth direction to an intermediate thickness of 153 mm followed again bypreheating to 395° C. for about 8 hours, and hot rolled in their lengthdirection to an intermediate thickness of 9 mm. Following hot rollingthe hot rolled intermediate products are heat treated by holding theproduct for 100 minutes at 395° C. followed by air cooling. In the nextstep material from ingot A is cold rolled in width direction inaccordance with the invention to an intermediate thickness of 7.6 mm,while material from ingot B is being cold rolled in its length directionto the same intermediate thickness. Subsequently ingot A has been coldrolled in its length direction to an intermediate thickness of 6.1 mm,and then to a final thickness of 4.6 mm. Between the cold rolling stepthe intermediate products are interannealed at 395° C. for 100 minutesfollowed by air cooling. Material from ingots B and C have first beencold rolled in their length and width direction respectively from 9 mmto 6.1 mm, heat treated and then cold rolled in its length directionfrom 6.1 to 4.6 mm. Subsequently both cold rolled material of ingot Aand B have been solution heat treated at 530° C. for 1 hours and thencooled to below is 150° C. by using air cooling allowing an averagecooling rate of about 0.3° C./sec, while the material from ingot Creceived the same treatment but has been solution heat treated at 480°C. for 1 hour. The cold rolled and solution heat treated sheets havebeen stretched at room temperature for 0.8% of their original length.Following stretching the sheet products have been aged in a three stepageing heat treatment, consisting of first 6 hours at 85° C., then 12hours at 120° C. and then 10 hours at 100° C. The processing steps arealso summarised in Table 2.

[0066] Following ageing the sheets have been tested for their mechanicalproperties as function of the direction, and for which the results arelisted in Table 3 and 4; all results are an average over three specimenstested. For the tensile testing the specimens had dimensions of:1_(o)=50 rm, b_(o)=12.5 mm, and d_(o)=4.6 mm. And further sheetsmaterials have been tested for their crack propagation characteristics,of which the results are shown in FIG. 1 for the T-L direction andcompared with the results of the master curve for 2024 material. FIG. 2shows the crack propagation characteristics for the L-T direction andcompared with the results of the master curve for 2024 material. Thematerials have also been tested for their thermal stability by holdingit for 300 hours at 95° C., after which the K_(co) has been tested inthe T-L direction only, the results of which are listed in Table 5.Further the sheet materials have been assessed on the presence ofLüder-lines, and it was found that both sheets materials from ingot Aand B were free from both Type-A and type-B Lüder-lines, while materialfrom ingot C showed presence of Type-A Lüder-lines.

[0067] From the results from Table 3 it can be seen that the materialmanufactured in accordance with the invention (ingot A and C) have muchmore isotropic mechanical properties than the material from ingot B.Further it can be seen that for ingot A and C material the proofstrength (PS) are higher for all direction. And the elongation asfunction of the testing direction are much more balanced with thematerial from ingots A and C than from material from ingot B, and wherethe balance for material of ingot A is better than for ingot C material.

[0068] From the results from Table 4 it can be seen that the fracturetoughness is increased with higher solution heat treatment temperatures.Further it can be seen that material manufactured with the methodaccording to the invention has even a somewhat further improved and morebalanced fracture toughness, which is likely to be due to the rollingpractice applied.

[0069] From the results from Table 5 it can be seen that the materialwhich have been solution heat treated at 530° C. (materials from ingot Aand B) have a good thermal stability, the results remain unchanged,while material solution heat treated at 480° C. shows an decrease inK_(co)-value of about 9%.

[0070] From the results of FIG. 1 for the critical T-L testingdirection, it can be seen that both materials have comparable or bettercrack propagation characteristics than 2024 material. Further it can beseen that material from ingot A gives better results than material fromingot B. Further it can be seen that for this critical testing directionthe resistance to crack propagation is improved with higher solutionheat treatment temperatures.

[0071] From the results of FIG. 2 for the L-T testing direction, it canbe seen that a higher solution heat treatment temperature cansignificantly improve the crack propagation resistance of the material.In this testing direction the material of ingot B shows better resultsthan material of ingot A and C, which is due to the rolling directionand which is in agreement with expectations. TABLE 1 Composition (weight%) Ingot Mg Li Mn Fe Si Zn Zr Sc Be A + C 4.90 1.65 0.18 0.08 0.05 0.590.08 0.08 0.001 B 4.70 1.50 0.22 0.08 0.04 0.70 0.05 0.05 0.002

[0072] TABLE 4 K_(CO) [MPa.✓m] for 400 mm CCT-panels Ingot L-T T-L A90.9 92.7 B 90.7 92.3 C 83.5 86.1

[0073] TABLE 5 K_(CO) [MPa.✓m] in T-L for 400 mm CCT-panels Ingot BeforeAfter A 92.7 92.7 B 92.3 92.3 C 86.1 80.1

[0074] TABLE 2 Process step Ingot A Ingot B Ingot C Preheat 395° C. for8 hours 1^(st) hot rolling In width direction to 153 mm Preheat 395° C.for 8 hours 2^(nd) hot rolling In length direction to 9 mm Anneal 395°C. for 100 minutes 1^(st) cold rolling width to 7.6 mm length to 6.1 mmwidth to 6.1 mm Interanneal 395° C. for 100 minutes 2^(nd) cold rollinglength to 6.1 mm length to 4.6 mm length to 4.6 mm Interanneal 395° C.for — — 100 min. 3^(rd) cold rolling length to 4.6 mm — — Solution heat530° C. for 1 hour 480° C. tr. for 1 hour Stretching 0.8% of originallength Ageing 85° C. for 6 hours/120° C. for 12 hours/ 100° C. for 10hours

[0075] TABLE 3 Ingot A Ingot B Ingot C PS [MPa] L 346 319 396 LT 325 306355 45° 272 231 311 UTS [MPa] L 449 476 476 LT 475 475 488 45° 429 391449 Elongation [%] L 7.6 8.0 5.8 LT 13.6 13.2 10.0 45° 17.6 28.2 16.0

Example 2

[0076] In a similar way as in Example 1 three ingots (ingots D, E and F)have been produced on an industrial scale, of which there is onemanufactured in accordance with the invention and two are manufacturedfor comparison. The chemical composition for all three ingots was thesame and is listed in Table 6, and had starting dimensions of350×1450×2500 mm. The processing route showed similarity with those ofExample 1 and are summarised in Table 7. Two different temperatures forthe solution heat treatment after cold rolling have been applied, viz.530° C. and 515° C.

[0077] Following ageing the sheets have been tested for their mechanicalproperties as function of the direction, and for which the results arelisted in Table 8 as function of solution heat treatment temperature;all results are an average over three specimens tested. For the tensiletesting the specimens had dimensions of::1_(o)50 mm, b_(o)=12.5 mm, andd_(o)4.6 mm.

[0078] From the results of Table 8 it can be seen that the materialmanufactured in accordance with the invention (ingot D) has much moreisotropic mechanical properties than the material from ingot E and F,more in particular the elongation is much more balanced. Further it canbe seen that the method in accordance with the invention results insignificantly higher proof strength levels. Further it can be seen fromthese results than a higher solution heat treatment temperature aftercold rolling results in higher mechanical properties after ageing. TABLE6 Composition (weight %) Ingot Mg Li Mn Fe Si Zn Zr Sc Be D/E/F 4.851.60 0.22 0.09 0.05 0.70 0.07 0.07 0.001

[0079] TABLE 7 Process step Ingot D Ingot E Ingot F Preheat 430° C. for8 hours 1^(st) hot rolling In length direction to 240 mm Preheat 395° C.for 8 hours 2^(nd) hot rolling In width direction to 9 mm Anneal 395° C.for 100 minutes 1^(st) cold rolling width to 7.6 mm length to 7.6 mmlength to 7.6 mm Interanneal 395° C. for 100 minutes 2^(nd) cold rollinglength direction to 6.1 mm length to 4.6 mm Interanneal 395° C. for 100minutes — 3^(rd) cold rolling length direction to 4.6 mm — Solution heat515° C. and 530° C. for 1 hour tr. Stretching 0.8% of original lengthAgeing 85° C. - 6 hours/120° C. - 12 hours/100° C. - 10 hours

[0080] TABLE 8 Ingot D Ingot E Ingot F 515° C. 530° C. 515° C. 530° C.515° C. 530° C. PS [MPa] L 346 362 298 283 301 289 LT 315 300 302 299306 297 45° C. 264 251 235 222 259 245 UTS [MPa] L 412 471 466 460 461450 LT 461 453 470 465 464 456 45° 412 403 387 375 421 413 Elongation[%] L 3.4 6.5 6.8 7.8 89 9.5 LT 10.3 11.1 15.1 15.6 16.3 15.4 45° 17.321.9 26.6 27.8 21.0 22.2

[0081] Having now fully described the invention, it will be apparent toone of ordinary skill in the art that many change and modifications canbe made thereto without departing from the spirit or scope of theinvention as set forth by the claims appended hereto.

1. Method for manufacturing of an aluminium-magnesium-lithium product, comprising the steps of subsequently: (a) providing an aluminium alloy consisting of (in weight %): Mg 3.0-6.0 Li 0.4-3.0 Zn up to 2.0 Mn up to 1.0 Ag up to 0.5 Fe up to 0.3 Si up to 0.3 Cu up to 0.3 0.02-0.5 selected from the group consisting of Sc 0.010-0.40_(—) Hf 0.010-0.25 Ti 0.010-0.25 V 0.010-0.30 Nd 0.010-0.20 Zr 0.020-0.25 Cr 0.020-0.25 Y 0.005-0.20 Be 0.0002-0.10

balance consisting essentially of aluminium and incidental elements and impurities; (b) casting the aluminium alloy into an ingot; (c) preheating the ingot; (d) hot rolling the preheated ingot to a hot worked intermediate product; (e) cold rolling the hot worked intermediate product to a rolled product in both the length and in the width direction with a total cold rolling reduction of at least 15%; (f) solution heat treating the cold rolled product in the temperature range of 465 to 565° C. for a soaking time in the range of 0.15 to 8 hours; (g) cooling the solution heat treated product from the solution heat treatment temperature to below 150° C. with a cooling rate of at least 0.2° C./sec; (h) ageing the cooled product to provide a sheet or thin plate product having a minimum yield strength of 260 MPa or more and a minimum tensile strength of 400 MPa or more in at least the L- and LT-direction, a minimum yield strength of 230 MPa or more and a minimum tensile strength of 380 MPa or more in the 45° to the L-direction, and further having a minimum T-L fracture toughness K_(co) of 80 MPa.{square root}m or more for 400 mm wide CCT-panels.
 2. Method in accordance with claim 1, wherein during step (d) the preheated ingot is hot rolled in both the length and in the width direction.
 3. Method in accordance with claim 1 or 2, wherein the Mg content is in the range of 4.3 to 5.5 weight %.
 4. Method in accordance with any one of the preceding claims, wherein the Li content is in the range of 1.0 to 2.2 weight %.
 5. Method in accordance with any one of the preceding claims, wherein the Zn content is in the range of 0.2 to 1.0 weight %.
 6. Method in accordance with any one of the preceding claims, wherein the provided aluminium alloy comprises at least Sc in a range of 0.01 to 0.08 weight %.
 7. Method in accordance with claim 6, wherein the provided product further comprises at least Zr in a range of 0.02 to 0.25 weight %.
 8. Use of the product obtained by the method in accordance with any one of claims 1 to 7 as aircraft skin.
 9. Use of the product obtained by the method in accordance with any one of claims 1 to 7 for the manufacture of aircraft lower wing skins.
 10. An aerospace airframe structure produced from an aluminium-magnesium-lithium product obtained by the method in accordance with any one of claims 1 to
 7. 11. Aircraft skin material produced from an aluminium-magnesium-lithium product obtained by the method in accordance with any one of claims 1 to
 7. 